Glyphosate is a known, effective herbicide. There are several organic ammonium salts of glyphosate useful as herbicides, including the methylamine salt and dimethylamine salt, and, as an example, monoalkylammonium and dialkylammonium (see, e.g., U.S. Pat. No. 4,405,531). Various glyphosate formulations are currently marketed, many of which are aqueous solutions that can be used as is or diluted prior to use.
Typically the glyphosate is provided as a salt that exhibits sufficiently high solubility in water to provide a high strength herbicidal formulation. For example, high strength formulations are known for the isopropylamine salt (IPA), the monoethanolamine (MEA) salt, and various formulations of the potassium (K) salt of glyphosate (see, e.g., U.S. Pat. No. 6,277,788; U.S. Pat. No. 6,365,551; WO 01/89302).
A high strength formulation is desirable for a variety of economic and environmental reasons. For example, it is desirable to provide a high strength formulation to reduce shipping and handling costs and to reduce the amount of packaging material that must be disposed. The high strength formulations are preferably stable and retain potency during storage and shipping. Furthermore, the high strength formulation are optimally provided as a clear, homogeneous liquid that is stable at temperatures at least as high as 50° C. and does not exhibit any precipitation at temperatures as low as 0° C.
However, high strength formulations can result in high viscosity. For example, formulations of the commonly used IPA salt of glyphosate become increasingly viscous at concentrations greater than 350 gram acid equivalent per liter (gae/L), particularly at concentrations greater than 440 gae/L. The high viscosity makes the formulation difficult to measure and pump, especially at the lower temperatures typically encountered at the beginning of the growing season.
The available glyphosate formulations typically include a surfactant. Inclusion of a surfactant can be desirable, as the resulting formulation can exhibit increased herbicidal activity or other improved characteristics. For example, glyphosate formulations are known to include alkylbetaine surfactants in combination with other surfactants (see, e.g., WO 03/067689).
A major limitation of the MEA and K salts of glyphosate is the incompatibility with a wide range of surfactants. In particular, polyoxyethylene alkylamines are only compatible with the MEA salt of glyphosate when the sum of the total average number of carbon atoms plus the average number of oxyethylene groups is equal to or less than 25 (see, e.g., U.S. Pat. No. 6,277,788). Similarly, many common surfactants are not compatible with the glyphosate K salt solution. For example, alkylamine ethoxylate surfactants are only compatible (i.e., allow a homogeneous mixture) when the degree of ethoxylation is no more than about 5, and such surfactants have a higher potential to cause eye irritation.
In addition, certain surfactants can interact with the glyphosate salt to increase the viscosity of the herbicidal formulation. For example, some of the surfactants in the oxyalkylene alkylamine class of compounds, when combined with the glyphosate salt, increase the viscosity of the formulation: If the viscosity is too high, handling of the concentrated herbicide can be difficult. Furthermore, highly viscous liquids are difficult to accurately measure, either for application to the plants or for dilution to a less concentrated spray solution. Depending upon the concentration and specific surfactant, the herbicidal formulation can form a gel, which makes most applications extremely difficult if not impossible to perform.
Dimethylamine is a flammable gas and generally provided as an aqueous solution at concentrations of about 40%. As such, it is difficult to obtain highly concentrated salts of glyphosate using only DMA solution as starting material. In laboratory conditions, the highest concentration of glyphosate prepared as DMA salt has been 560 gae/L (grams acid equivalent per liter), even fully loaded with surfactant.
In light of the above described problems, there is a continuing need for improvements in the relevant field including improved high strength herbicidal formulations that exhibit low viscosity and suitable efficacy. The present invention addresses these needs and provides a wide variety of benefits and advantages.